Conventional tire inflation valves comprise a tubular valve stem that is internally threaded at one end to receive a valve core. In use, the valve stem extends through a so called valve aperture in the wheel rim and provides an inflation passage for the admission of air to the interior of the tire. In the case of tubeless tires the stem is sealed to the wheel rim about that aperture. If the valve is a so called "snap-in" valve, the seal is effected by virtue of the tight fit within the valve aperture of a basal component of the stem, usually an elastomeric collar encircling and bonded to the stem. If the valve is a so called "clamp-in" valve, the seal is effected by means of annular clamping means including a ring nut and sealing washers or the like encircling the stem and securing it to the rim. In the case of tubed tires the valve stem is bonded to, or in some instances clamped to, the tube wall to provide an inflation passage therethrough.
In all instances, the valve core comprises a valve body incorporating a seat and a spring-loaded valve element co-acting with that seat, and constitutes a non-return valve controlling air flow through the inflation passage. The valve core is a replaceable item and it and the part of the valve stem that receives it have become standardised throughout the world insofar as their cross-sectional dimensions are concerned.
It is well known that tire life and vehicle safety depend largely on tires being maintained at the recommended inflation pressure, which varies with the vehicle-tire combination, tire location on the vehicle and operating conditions. It is also a fact of life that many persons omit to check tire pressures regularly, and indeed frequently are not certain as to what is the correct pressure in a particular instance.
Thus a number of devices have been proposed hitherto for providing a continuous indication of the correctness or otherwise of the pressure in a tire; so as to remove the need for deliberate actions on the part of the motorist, after the initial inflation to the correct pressure, to check the existing pressure and the need for the motorist to remember or determine the correct value.
Such prior known devices have included indicators adapted to be secured to the free or exposed end of a conventional valve stem in lieu of a conventional dust cap thereon. Such self contained, auxiliary indicators necessarily hold the core's valve element off its seat to enable the tire pressure to reach and act on the indicator. Unfortunately such indicators have proved to be unreliable in maintaining a seal between themselves and the valve stem. Thus they frequently allow air to leak from the tire, and have not found wide spread acceptance.
As a better alternative, various indicating valves have been proposed incorporating means within the valve stem to enable the valve itself to provide an indication of the inflation pressure. Typically such indicating valves have comprised a two part stem, namely an outer stationary part and an inner movable part telescopically slidable relative to the stationary part, sealing means to prevent leakage from between the two parts and yieldable reaction means counteracting the tire pressure acting upon the movable part. Thus the rest position of the movable part of the stem depends on the tire pressure, and the design is such that departure from a rest position corresponding to the correct pressure is made manifest to an observer.
One class of prior known indicating valves have utilized sliding sealing means wherein a seal makes sliding contact with one or other of the parts of the stem. It has been found however that sliding seals are a cause of leakage. Furthermore they cause inaccuracy in operation due to wide variation in the frictional restraint imposed by the seal upon the movable part of the stem, due, inter alia, to deterioration in the condition of the contacting sealing surfaces as a result of aging, contamination or corrosion.
To overcome the inherent difficulties with sliding seals it has also been proposed to use non-sliding sealing means comprising a flexible barrier, sealed to each stem part and extending from one to the other to maintain a seal therebetween while allowing the moving part to move.
Prior art proposals exemplifying pressure indicating inflation valves utilising a reaction spring to determine the rest position of a movable stem part in combination with flexible barrier type sealing means are illustrated by the specifications of U.S. Pat. Nos. 3,230,968 (Struby) and 3,811,459 (Schmidt), while an example of a valve wherein all of the reaction force is supplied by similar, but resilient, sealing means is that of U.S. Pat. No. 5,027,848 (van Leeuwen).
From those specifications it will be seen that prior known flexible barrier seals fall into two classes, namely axially extending, concertina type tubes or boots enclosing the reaction spring (for example, Struby's FIGS. 1 to 4 embodiment), and more or less radially extending diaphragms (for example, Schmidt's preferred embodiments and Struby's FIGS. 5 and 6 embodiment).